Recent years have seen increasing high performance in electronic devices such as mobile information devices and information appliances following the development of digital technology. With the increased high performance in these electronic devices, there has been rapid progress in the increase in scale, level of integration, and speed of nonvolatile memory devices to be used, and the uses for such memory devices are also expanding rapidly. Among these, applications for large-capacity nonvolatile memories represented by a flash memory are rapidly expanding. In addition, as a next-generation nonvolatile memory to replace the flash memory, research and development on a nonvolatile memory device which uses a variable resistance nonvolatile memory element (also called variable resistance element) is advancing. Here, variable resistance element refers to an element which has a property in which its resistance value reversibly changes according to electrical signals, and is capable of storing information corresponding to the resistance value in a nonvolatile manner.
As an example of such variable resistance element, there is proposed a nonvolatile memory device having a variable resistance layer in which transition metal oxides of different oxygen content atomic percentages are stacked. For example, Patent Literature (PTL) 1 discloses selectively causing the occurrence of oxidation/reduction reaction in an electrode interface which is in contact with a variable resistance layer having a high oxygen content atomic percentage, to stabilize resistance change.
The aforementioned conventional variable resistance element includes a lower electrode, a variable resistance layer, and an upper electrode, and a memory array is configured from a two-dimensional or three-dimensional array of such variable resistance element. In each of the variable resistance elements, the variable resistance layer is of a stacked structure including a first variable resistance layer and a second variable resistance layer, and the first and second variable resistance layers comprise the same type of transitional metal oxide. The oxygen content atomic percentage of the transitional metal oxide forming the second variable resistance layer is higher than the oxygen content atomic percentage of the transitional metal oxide forming the first variable resistance layer.
By adopting such a structure, when voltage is applied to the variable resistance element, most of the voltage is applied to the second variable resistance layer which has a high oxygen content atomic percentage and exhibits a higher resistance value. Furthermore, oxygen, which can contribute to the reaction, is abundant in the vicinity of the interface. Therefore, oxidation/reduction reaction occurs selectively at the interface between the upper electrode and the second variable resistance layer, and stable resistance change can be realized.
Furthermore, in the above-described conventional variable resistance element, in order to transition to a state in which resistance change is started, it is necessary to initially apply voltage to the variable resistance element to form a filament in the second variable resistance layer (such filament formation is referred to as break, and the voltage applied to the variable resistance element at such time is referred to as an initial break voltage). In view of this, there is proposed a nonvolatile memory device in which a step is formed in a variable resistance element to allow easy formation of an initial filament. For example, in PTL 2, a step is formed in the interface between a first variable resistance element and a second variable resistance element, and the second variable resistance layer is formed to cover the bend of the step. By adopting the above-described configuration, the shape of the step of the first variable resistance layer is reflected, thereby forming the bend in the second variable resistance layer on the step, and thus it is possible, through electric field concentration, to cause the break phenomenon even with a low voltage, with the bend as a starting point.